Electric initiator



L. A. BURROWS ET AL ELECTRIC INITIATOR Jan. 2, 1940.

Filed April 14, 1937 ATTORNEY.

Patented Jan. 2, 1940 UNETEQ STATES PATENT FFHCE ELECTRIC m'rm'roa Lawton A. Burrows, Wenonah, and Walter E. Lawson, Woodbury, N. J., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del.', a corporation of Delaware Application April 14, 193': Serial No. 136,743

12 Claims. 01. 102-10) This invention relates to electric explosive are of particular importance, for example, in initiators generally and more particularly to seismographic prospecting, where the need for electric blasting caps possessing superior initiataccuracy in recording the exact time of detonaing features and of a design adapted to promote ation of the cap makes it essential that there safetyin handling and use be practically no time interval between the Electric blasting caps consist essentially of a. breaking of the bridge wire and the actual cylindrical metal shell containing a detonating detonaticn of the cap compositions, in other base charge, an ignition composition, and an words that there shall be no appreciable inducelectric firing means in contact with, or embedtion period. The copending case of Aughey,

0 ded in, the ignition composition. The firing Burrows, and Lawson (Serial No; 47,385, filed means commonly employed comprises an elec- October 30, 1935), has described various ignition trical conductor of high resistance joining the compositions th r hi h y satisfactory f two leg wires of the firing circuit. When the utilization in caps characterized by absence of current is applied to the firing circuit, this high time lag. for xample ad s yphn t bas ad resistance bridge wire is heated to incandescence, p c a OQPD acetylide, S11v6! @Zide, and the thereby firing the ignition composition, which in l e. Suc CO P however, have h turn initiates the detonating basecharge. themselves to be susceptible to the action of ec ic blasting caps may be classified into static electricity, and for this reason are hazardthree general types depending on certain strucous for' use under ordinary conditions. The

tural characteristics of the firing means. One above-mentioned app c discloses methods 20 of these is known as the bridge plug type and o render ng c ps Containing Such Compounds contains a plug of insulating material, usually in entirely free from hazard from' electro-static the shape of a truncated cone,-which holds the causes. a leg wires of the firing circuit in fixed relationship. While employ of rapid ignition compounds The leg wires, extending somewhat beyond the has been made entirely practicable and free from base of the plug, are connected by a high resisthazard, there have been some undesirable feaance bridge wire which is surrounded by an ignitures in the electric blasting caps containing tion composition. such compounds from the point of view of lack A second type of electric blasting cap is known of simpl c in Construction A marked as the concave plug type. In this type the leg vance has been made in the direction of providwires are also held in fixed spaced relationship ing an instantaneously functioning blasting cap by a conically shaped plug of insulating material. free from susceptibility to static, but further ad- In this case, however, the smaller base of the plug vances have been indicated to be desirable. is provided with a shallowiconcavity. The ends The .object of the present invention is a new of the leg wires, and the bridge wire connecting and improved electric blasting initiator which is the two, are arranged within this cavity, which free from susceptibility to electrostatic disserves to protect the bridge wire from rupture. charges. A f t er j t i l electric blasting The ignition composition employed for this type cap without an appreciable induction period and of blasting cap is usually applied in plastic condisubstantially free from susceptibility to static tion and dries to form a hard cemented compoelectricity. A still .further object is an electric sition in this depression. blasting cap of the character described, in which The third type of electric blasting cap is known the means of ignition has been simplified and as the match-head type and consists generally improved. Other objects will be apparent as the of a fiat strip of insulating material, the two invention is described more fully hereinafter. faces of which are covered with metal foil. A We have found that the foregoing objects are bridge wire passing around the end of the strip accomplished by the employment of an ignition connects the two separated pieces of foil, which means comprising ahead or mass of ignition are soldered to therespective leg wires. A small material attached on 'the bridge wire of the pelleted mass is usually formed about the bridge bridge plug type of initiator, said bead being wire to serve as ignition means. confined within a surface layer of hard, sub- 50 The above three types are applicable to all uses stantially non-flexible, pressure-resistant coating where electric blasting caps are desired, for exmaterial. While such material may be of various ample in ordinary blasting operations and in compositions, it should be one that can be applied cases where speed of functioning and absence of to fluid form/and become hard, non-flexible, and

time lag are requisites. The so-called iast caps pressure-resisting on standing. ,It maytfor x- ,65

lmple, be a solution or dispersion of a coating naterial in a volatile solvent of such nature that he latter may be subsequently volatilized. Pref- :rably, however, we use as coating material one hat can be applied in molten form and that will iecome solidified and hardened on cooling. It is lesirable, therefore, that the material have a :harp melting point and sharp solidification point and that it possess a considerable degree of hard- Jess and suificient strength to offer satisfactory :onfinement to the explosive charge. This molten method of application is much more desirable than the solvent method first mentioned, for the reasons that it avoids the difficulty of removing the solvent, and likewise avoids the unsatisfactory nature of the coating, which exists as long as any solvent remains therein.

As an example of such a material we-may use a resinous composition, for example a cumarone or cumarone-indene resin, a desirable one being of the latter composition and melting at around 150 to 160 C. Another example of a suitable resin is one of the chlorinated biphenyl type such as have been produced commercially under the trade name Aroclors, preferably one melting between 100 and 150 C. We may also apply a coating of a hard wax, such as carnauba wax, or carnauba wax containing a filling material. Another efiective type of coating from the point of view of confinement comprises a fusible metal, for example Woods metal, having a melting point of about 90 C. Preferably, we employ as coating material a composition comprised largely of sulfur, and desirably sulfur to which various other materials have been added in small amounts for control of the consistency and strength. A composition containing about 95% sulfur is suitable, for example, with additions of such materials as graphite, pitch, chalk, siliceous materials, and the like.

The above coatings are applied to the head in molten form. A casting method may, for ex-' ample, be used, where the mold is so shaped that it can be withdrawn from the hardened bead. Preferably, we perform the coating operation by dipping the hard explosive bead into a bath of the molten coating material, withdrawing it, and allowing the bead to solidify and harden on coolmg.

As ignition material we prefer to use lead styphnate or analogous compound that ignites substantially instantaneously, for example basic lead picrate, silver azide, the silver or mercury salt of chlorinated azodicarbonamidine, and the like. A suitable form for application to the bridge wire is a suspension of the ignition composition in a viscous solution of nitrocellulose in a suitable solvent. Using sucha suspending solution, a bead is formed that smooths outsatisfactorily about the bridge wire in the form of a small drop and sets quickly to a hardened mass. This bead is covered with a surface coating of a hard, confining, pressure-resistant material that is rigid when cold. Such a coating may be comprised desirably of the sulfur composition described heretofore, containing, for example, over 90 or 95% sulfur. The composition may be applied by maintaining it in molten form at around 150 C., for example, and dipping therein the bead of ignition composition, the important consideration being that the bead be completely covered. The presence of such a coating has the advantages of (1) protecting the friction-sensitive bead from any possible impact hazards resulting during the spacing of the head in the cap, (2) pr te g the bead from electrostatic discharges, and (3) rfinement of the coating is suflicient to cause detonation of the enclosed charge and that ignition of the secondary charge is brought about then by shock rather than by flash. Reduction or elimination of the time lag is thereby accomplished. Whereas compounds such as lead styphnate are so fast that, when used in quantity, they detonate substantially instantaneously, the amount present in the bead is so small as to defiagrate when unconfined. The confinement of the coatings described is sufiicient to bring practically instantaneous detonation rather than defiagration. When such compounds as lead styphnate and the like are used in initiators of the concave plug type, elimination of time lag is attained without use of a confining coating because of the confinement afforded by the concavity and the juxtaposed charge.

While our invention is particularly applicable when fast, static-susceptible compounds are used as bead ignition materials, it gives decreased induction periods also when compounds are used of a slower order of ignition, for example, mercury fulminate.

We preferably impart additional protection by so constructing the firing mechanism that any electrostatic charge will be discharged outside the locus of the ignition composition. While various means for this result may be adopted, we find a simple and effective method to comprise bending the two leg wires in toward one another shortly beyond the point Where they leave the bridge plug. By this expedient the wires are closer to the shell wall near the plug than they are atthe locus of the bridge wire and the bead surrounding said bridge wire. Consequently, any static charge will tend to discharge at the point where said leg wires are closest to the metal shell wall, thereby protecting the sensitive ignition composition.

By way of more exact illustration, the atwhile above the bridge plug are the water-proofing composition l2, and a sulfur seal l3. The leg wires are connected at their termination, below the bridge plug, by a high resistance bridge wire I which is surrounded by the bead 8, having an inflexible pressure-resistant coatings. A feature that adds to the protection against electrostatic effects may be illustrated on leg wire 5 which leaves the bridge plug at "I. At II this wire is distinctly bent in toward the center of the cap shell and away from the shell wall. In case of discharges of static electricity, therefore, such discharge will take place to the shell wall from the portion In to II, this section being sufficiently outside thelocus of the bead of sensitive ignition material as to afford complete protection for the cap. It will be seen that the same protection is likewise provided by wire 6. Only holds in spaced position the leg wires 5 and 6, I

when the firing current is applied to the leg wires will bead i be ignited, which will cause instantaneous detonation of the other cap charges.

From the foregoing description, it will be apparent that one feature of our invention is the discharging portion of the firing circuit, with respect to possibly accumulated charges of static electricity. In the example, there is the double medium for discharge, by reason of the two wires, and a construction such as is shown there is our preferred form. It will be understood, however, that other means for facilitating dis charge may be included if desired, for example attachments to, or distortions of, one or both wires bringing them closer to the shell wall, or even the grounding of one of the wires to the metal wall. The bead of ignition material will preferably be positioned close to the layer of the charge to be ignited.

The following table is illustrative of the advantages obtained by our invention, where the induction period for bead type ignition caps is shown as measured by the cathode ray oscillograph. Examples are given of coated and uncoated beads, in which lead styphnate is used as an example of a very fast compound and mercury fulminate as illustrating one of the detonating compounds in ordinary commercial use.

The foregoing results indicate the value of a firm, non-flexible, pressure-resisting coating of the type described, in decreasing the induction period in electric blasting caps employing the bead of ignition.

The advantages of our invention will be apparent, therefore, since an improved electric blasting initiator has been disclosed having increased safety properties and of a simplified form of construction for an initiator embodying such features. While the invention has been described particularly in relation to electric blasting caps, which contain detonating explosives and are adapted to the initiation of high explosives of the dynamite type, it is equally applicable to the so-called electric squibs, which are adapted for use in igniting defiagrating explosives such as black powder.

In the foregoing detailed description of our invention it is apparent that many variations in composition and procedure may be made without departing from the spirit or scope thereof. We wish to be limited, therefore, only by the following patent claims: Y

We claim:

1. An electric blasting initiator comprising a loaded shell, a plug disposed in said shell, a pair of leg wires extending into said shell through said plug with their ends disposed below said said plug, an electric filament disposed between the ends of said leg wires electrically connecting said ends, a coated bead of ignition material spaced from the said plug and suspended on said filament, the coating on said bead comprising a hard, substantially non-flexible pressure-resistant material of a hard, solidified melt completely coating said bead of ignition material, said coating material being characterized by a sharp solidification point above normal temperatures.

2. An electric blasting initiator comprising a loaded shell, a plug disposed in said shell, a pair of leg wires extending into said shell through said plug with their ends disposed below said plug, an electric filament disposed betweenthe ends of said leg wires electrically connecting said ends, a coated bead of ignition material dis-.

posed about said filament, the coating on said bead comprising a hard, substantially non-flexible pressure-resistant layer of a hard, solidified melt completely surrounding said bead of ignition material, said coated bead being spaced from the plug and from the sides of the shell, said coating material being characterized by a sharp solidification point above normal temperatures.

3. An electric blasting initiator comprising a loaded shell, a bridge plug disposed within said shell, bridge posts extending through and below the base of said bridge plug, a bridge wire disposed between said bridge posts below said bridge plug, a coated bead of ignition material disposed about said bridge wire and spaced from the plug and shell wall, said coating comprising a hard, substantially non-flexible pressure-resistant layer of solidified sulfurous composition.

4. The electric blasting initiator of claim 3 wherein the bead of ignition material comprises lead styphnate.

5. The electric blasting initiator of claim 3 wherein the bead of ignition material comprises mercury fulminate.

6. The electric blasting initiator of claim 2 wherein the coating material comprises a resinous material.

7. The electric blasting initiator of claim 2 wherein the coating material comprises a readily fusible metal fused about said head to ,form said hard, substantially non-flexible pressureresistant coating.

8. The electric blasting initiator of claim 3 wherein the sulfurous composition comprises approximately 95% sulfur.

9. An electric blasting initiator comprising a charged metal shell and a firing circuit, said circuit comprising a bridge plug, leg wires passing through said bridge plug in dielectric relation with each other, a bridge wire connecting said leg wires, and a bead of ignition material surrounding said bridge wire, said bead being con fined within a hard surface layer of substantially non-flexible, pressure-resistant material, said leg wires being bent in toward one another below the region of their emergence from said bridge plug, and outside the locus of said bead of ignition material, permitting the harmless discharge of static electricity.

10. An electric blasting initiator comprising a charged metal shell and a firing circuit, said circuit comprising a bridge plug, leg wires passing through said bridge plug in dielectric relation with each other, a bridge wire connecting said leg wires, and a bead of ignition material comprising lead styphnate surrounding said bridge wire, said bead being confined within a hard surface layer, of substantially non-flexible, pressure-resistant material, said leg wires being bent in toward one another below the region of their emergence from said bridge plug, and outside the locus of said bead of ignition material, permitting the harmless discharge of static electricity.

11. An electric blasting initiator comprising a charged metal shell and a firing circuit, said circuit comprising a bridge plug, leg wires passing through said bridge plug in dielectric relation with each other, a bridge wire connecting said leg wires, and a head of ignition material comprising basic lead picrate surrounding said bridge wire, said head being confined within a hard surface layer of substantially non-flexible, pressureresistant material, said leg wires being bent in toward one another below the region of their emergence from said bridge plug, and outside the locus of said bead of ignition material, permitting the harmless discharge of static electricity.

12. An electric blasting initiator comprising a charged metal shell anda firing circuit, said cira eaaro cult comprising a bridge plug, leg wires passing through said bridge plug in dielectric relation with each other, a bridge wire connecting said leg wires, and a bead of ignition material comprising silver azide surrounding said bridge wire, said bead being confined within a hard surface layer of substantially non-flexible, pressure-resistant material, said leg wires being bent in toward one another below the region of their emergence from. said bridge plug, and outside the locus of said bead of ignition material, permitting the harmless discharge of static elec-- tricity.

LAWTON A. BURROWS. WALTER E. LAWSON. 

